The size of the ball of the natural hip joint (femoral head) varies and usually ranges
from 40 to 54 mm (with smaller sizes in females). During the initial development
of hip replacement, the surgeons tried to make the implants which mimic the size
of the natural femoral head. However, it was soon realised that when a large metal
ball articulates with a plastic cup, the plastic part gets worn out quickly due to
“volumetric wear”. To decrease the "wear" of the plastic, Sir John Charnley (who
was one of the pioneers in the development of modern hip replacements), opted for
a smaller 22 mm metal head (which is much smaller than the natural femoral head)
in the design of Charnley hip replacement. This design was the commonest hip replacement
done in the UK until recently.

Studies have shown that with this technique, there were fairly good long term results
in patients who are not very young and active. However, there were some restrictions
placed on what the patients can do to avoid dislocations (when the ball comes out
of the joint) and a high dislocation rate following hip replacement has been quoted
with smaller “ball size” in multicentre studies1.

Many surgeons over the past few years have started using slightly larger size femoral
head (28 mm) which is still within the tolerance of conventional metal on plastic
bearing. However, still it is smaller than the natural femoral head. The problem
of wear and dislocation is still a potential concern.

In a recently published data from UK National Joint Registry, dislocation still remains
a major reason (17%) for revision of hip replacement (NJR data 2008).

A study2 in which Mr.Ganapathi was involved found that the financial cost of treating
dislocated hip replacements was high (£500000 on a conservative estimate in dealing
with 100 first time dislocators). In addition many patients had to undergo further
revision surgeries and the functional results were less than optimal. While the cause
of hip dislocations is multifactorial, using a larger ball could decrease the incident
of dislocations. However, the size of the ball was limited by the material properties.

In addition, much younger and active patients are being offered hip replacements
now. The expectations of young and active patients are much higher and they would
like to go back to their active lifestyle with little restrictions. To improve the
durability of the hip replacements, material scientists have come up with modern
solutions (alternative bearings).

There are essentially three types of alternative materials – cross linked poly (plastic),
ceramic and metal. These alternative bearings also allow larger size ball to be used
32 mm, 36 mm and even 40 mm. However, the size would be limited by the cup (socket
size) and also some material allow slightly bigger ball to be used with the same
cup size compared to other materials.

Lab studies and clinical studies also support the concept that the larger sized balls
decrease the dislocation3,4. A study conducted by Mr.Ganapathi also found that the
larger size ball also decrease the risk of dislocations even in complex revision
surgery compared with historical studies5.

However, because of the relatively new concept, the "modern" alternative bearings
do not have long term results although the evidence in short term data and lab studies
are encouraging. In addition, although the alternative materials are stronger and
has more wear resistance, they are also relatively brittle and have somewhat poor
tolerance to variation in the cup (socket) placement. They also have their special
complications including fracture of the plastic or ceramic and rarely squeaking.

Metal on metal bearings, in addition also allow the surgeon to use an even larger
size ball (same size as the natural femoral head – “anatomical sized head” or “large
diameter head”). This can be done as a part of conventional total hip replacement
or hip resurfacing. Because the size of the ball is almost the same as the natural
femoral head, there is almost a negligible rate of dislocation and very little restriction
in activities once the soft tissues have healed. Data from gait analysis show better
gait pattern with larger diameter balls 6,7 and better patient reported outcome scores8.

A very recent study from Montreal9 in which Mr.Ganapathi was involved analysed the
range of movement following different types of hip replacement using standardised
digital photographs and computer software.

This study showed that the range of movement was better with the “large diameter
head” total hip arthroplasty when compared with 28 mm head total hip arthroplasty
or hip resurfacing but still was less than the range of movement of the normal contralateral
hip. In addition, functional score (WOMAC score) also correlated with range of movement.

Although, large head metal on metal type hip replacements/hip resurfacing appear
to be the ideal bearing option with regards to stability and range of movement, recent
studies have indicated that a small proportion of these patients may develop possible
reaction to the metal debris or ions and this can lead to loss of bone and loosening
requiring further complex revision surgery. This is a more common occurrence in females
and smaller components although the reasons are not entirely clear. Component positioning
also appears to play a role. More recently, the effect of corrosion between the taper
junction is also considered to be a possible factor. However, not all metal on metal
bearings are the same and they vary in subtle design features which might influence
the metal ion levels and outcome. Very recently one of the designs has been recalled
by the manufacturer because of higher than expected failure rate. Further innovations
are being developed by some implant companies to decrease the metal ion levels (see
below - Newer Developments).

In view of the recently recognised metal debris related problem, it is important
to monitor patients who have metal on metal bearings regularly. In April 2010 MHRA
(Medicines and Healthcare products Regulatory Authority) issued an alert to that
effect (please click the following link for the alert).

Thus although majority of patients will probably have a good result following hip
resurfacing with careful patient selection and correct technique, a small proportion
may develop complications related to the metal on metal bearing and ultimately it
is a balance between the benefits and risks.

Newer Developments: To maintain the advantage of the large diameter bearing (better
range of movement and better stability) while minimising the potential adverse effects
of metal ions, implant companies are adopting various strategies. They include surface
engineering of the metal surface with ceramic (which has been shown in a recent study
to have very low metal ion levels because of the ceramic surface modification) and
using ceramic itself as large diameter bearing option. While the early results appear
promising, it is important to understand that as they are recent developments, no
long term results are yet available. Mr.Ganapathi would be happy to discuss with
you regarding these newer developments and would be able to offer those options if
appropriate.

1. Outcome of Charnley total hip replacement across a single health region in England:
the results at five years from a regional hip register. J Bone Joint Surg Br. 1999
Jul;81(4):577-81.

Fender D, Harper WM, Gregg PJ.

Using a regional arthroplasty register we assessed the outcome at five years of 1198
primary Charnley total hip replacements (THRs) carried out in 1152 patients across
a single UK health region in 1990. Information regarding outcome was available for
1080 hips (90%) and 499 had an independent clinical and radiological assessment.
By five years the known rate of aseptic loosening was 2.3%, of deep infection 1.4%,
of dislocation 5.0% and of revision 3.2%. The radiological assessment of 499 THRs
revealed gross failure in a further 5.2%, which had been previously unrecognised.
The combined rate of failure of nearly 9% is higher than those published from specialist
centres and surgeons, but is probably more representative of the norm. Our study
supports the need for a national register and surveillance of THRs. It emphasises
that all implants should be followed, and suggests that the results of such surgery,
when performed in the general setting, may not be as good as expected.

We have studied the natural history of a first episode of dislocation after primary
total hip replacement (THR) to clarify the incidence of recurrent dislocation, the
need for subsequent revision and the quality of life of these patients. Over a six-year
period, 99 patients (101 hips) presented with a first dislocation of a primary THR.
A total of 61 hips (60.4%) had dislocated more than once. After a minimum follow-up
of one year, seven patients had died. Of the remaining 94 hips (92 patients), 47
underwent a revision for instability and one awaits operation (51% in total). Of
these, seven re-dislocated and four needed further surgery. The quality of life of
the patients was studied using the Oxford Hip Score and the EuroQol-5 Dimension (EQ-5D)
questionnaire. A control group of patients who had not dislocated was also studied.
At a mean follow-up of 4.5 years (1 to 20), the mean Oxford Hip Score was 26.7 (15
to 47) after one episode of dislocation, 27.2 (12 to 45) after recurrent dislocation,
34.5 (12 to 54) after successful revision surgery, 42 (29 to 55) after failed revision
surgery and 17.4 (12 to 32) in the control group. The EuroQol-5 dimension questionnaire
revealed more health problems in patients undergoing revision surgery.

The size of the femoral head and acetabular anteversion are crucial for stability
in total hip replacements. This study examined the effects of head diameter and acetabular
anteversion on the posterior instability after total hip replacement in an in vivo
setting. The acetabular shell was inserted at 0-20 degrees of anteversion at five
degree intervals. By using different head sizes (28 mm, 32 mm, 36 mm), the degrees
of dislocation were recorded by computer navigation. The 36-mm group consistently
showed better stability compared with the 32- and 28-mm groups, regardless of the
degree of cup anteversion. Within each group of head size, the hip was significantly
more stable when the cup anteversion increased from 0 degrees to 10 degrees . The
difference became insignificant when it increased from 15 degrees to 20 degrees.

Dislocation after total hip arthroplasty (THA) remains a problem despite many advances
in technique and prosthetic design over the 5 decades since the introduction of total
joint replacement. This article reports the short-term results (1 year of follow-up)
of THA in 235 patients who received a large, anatomically sized femoral head (BFH
Technology; Wright Medical Technology, Inc, Arlington, Tennessee) with modular necks
for hip stability. The prosthesis allows a 6-mm differential between the size of
the acetabular component and femoral head size. Patients also received a Conserve
monoblock acetabular cup and a Profemur femoral stem (Wright Medical Technology,
Inc) implanted without cement. Postoperative clinical evaluations included measurements
of Harris Hip Scores and range of motion, along with assessments of pain and function
and radiological evaluations. There were no complications (deep venous thrombosis,
pulmonary embolism, infection, reoperations) and no dislocations. All clinical evaluations
showed statistically significant improvement (P<.001) at 1-year follow-up, and radiographic
evaluation has shown no evidence of osteolysis or implant loosening. This study indicates
that using a large femoral head may reduce the incidence of dislocation and may enable
early return to activities postoperatively. Future evaluations of this patient group
will elicit longer-term follow-up data.

5. The use of large diameter femoral heads in revision total hip replacement

Aim: To investigate the outcome following revision total hiparthroplasty (THA) using
36 mm and 40 mm modular femoral heads.Methods: Details were retrieved from our
arthroplasty databaseregarding all revision THAs done in our unit using 36 mm and40 mm femoral heads. Follow-up information was obtained frompatient records and
telephone conversation.Results: The cohort considered totalled 107 revision THAs,
93using a 36 mm head and 14 using a 40 mm head. All received eitherhighly cross-linked
UHMWPE liners or metal on metal liners.The indications for revisions were recurrent
instability ineight, periprosthetic fracture in 11, second stage revisionin 24,
fracture of the femoral stem in one and aseptic looseningin the remaining 63. At
a minimum follow up of one year, informationwas not available for five but they
did not have any recordof dislocation. Out of the remaining 102 patients, dislocationoccurred in 4 hips (3.9%). None of the revisions done with 40mm head dislocated.
In two of the dislocations, the initialindication for revision THA was recurrent
instability and ifthey are excluded, the dislocation rate was 1.96%.Discussion:
Dislocation and the sequalae of recurrent instabilityremains a significant problem
following revision THA and theexisting literature varies greatly in the quoted dislocationrates. We believe that the use of 36 mm and 40 mm femoral headsin our unit has been
a major factor in low (3.6%) dislocationrate following revision THA. To date there
have been no problemsencountered resulting from the use of highly cross-linked UHMWPE.

BACKGROUND: Primary total hip arthroplasty leads to better functional capacities
but a general weakness of abductor muscles often persists. A larger head component
may improve the postural balance in the medial-lateral direction. The aims of this
study are (1) to compare postural stability in patients after total hip and surface
replacement arthroplasties and (2) to evaluate the effect of the biomechanical reconstruction
on postural stability. METHODS: Six months post-surgery, three groups of ten subjects
(total hip and surface replacement arthroplasties and control) performed quiet standing
tasks in both dual and one leg stance and a hip abductor muscles strength test. The
root-mean-square amplitude of centre of pressure and centre of mass displacement
in the anterior-posterior and medial-lateral directions were calculated for dual
stance task. FINDINGS: Statistical analyses showed greater centre of pressure and
centre of mass displacement amplitude in the medial-lateral direction during the
dual stance for the total hip arthroplasty compared to the surface replacement and
control subjects (P<0.05). All control subjects completed the one leg stance compared
to nine in the surface replacement and five in the total hip arthroplasty group.
No statistical difference was found between the groups in the hip abductor muscles
strength. INTERPRETATION: The better anatomical preservation, absence of femoral
stem and the larger bearing component could account for the return to better postural
stability in surface replacement patients in comparison to total hip patients. Further
studies are needed to determine the impact of each of these factors on the postural
balance.

OBJECTIVE: To compare postural balance between patients who have had either a large
diameter head total hip arthroplasty or surface replacement arthroplasty. DESIGN:
Observational study. SETTING: Outpatient biomechanical laboratory. PARTICIPANTS:
Two groups of 14 patients with surface replacement or large diameter head total hip
arthroplasties recruited from a larger randomized study and 14 control subjects.
INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Postural balance during quiet
standing in dual and one-leg stance (operated leg), hip abductor muscle strength,
clinical outcomes, and radiographic analyses were compared between groups. RESULTS:
Compared to the control group, patients in both groups showed smaller center of pressure
displacement amplitude in the medial-lateral direction in dual stance. Patients with
large diameter head total hip arthroplasty showed lower hip abductor muscle strength
compared to control subjects. There was statistical difference between the 2 patient
groups in biomechanical reconstruction of the hip. Despite these differences, there
was no significant difference in the ability to complete the one-leg stance task
between the 3 groups. CONCLUSIONS: The muscular strength in the operated limb could
be mainly responsible for the lower center of pressure displacement amplitude compared
to control subjects. However, the ability to complete the one-leg stance demonstrates
that patients do not fear to load the hip prosthesis when needed. The large diameter
femoral head may be a major mechanical factor contributing to these results.

8. Comparison of patient-reported outcomes between hip resurfacing and total hip
replacement.

This study compared the demographic, clinical and patient-reported outcomes after
total hip replacement (THR) and Birmingham Hip Resurfacing (BHR) carried out by a
single surgeon. Patients completed a questionnaire that included the WOMAC, SF-36
scores and comorbid medical conditions. Data were collected before operation and
one year after. The outcome scores were adjusted for age, gender, comorbid conditions
and, at one year, for the pre-operative scores. There were 214 patients with a THR
and 132 with a BHR. Patients with a BHR were significantly younger (49 vs 67 years,
p < 0.0001), more likely to be male (68% vs 42% of THR, p < 0.0001) and had fewer
comorbid conditions (1.3 vs 2.0, p < 0.0001). Before operation there was no difference
in WOMAC and SF-36 scores, except for function, in which patients awaiting THR were
worse than those awaiting a BHR. At one year patients with a BHR reported significantly
better WOMAC pain scores (p = 0.04) and in all SF-36 domains (p < 0.05). Patients
undergoing BHR report a significantly greater improvement in general health compared
with those with a THR.

9. Range of motion of large head total hip arthroplasty is greater than 28 mm total
hip arthroplasty or hip resurfacing.

Background: Reduced range of motion of the hip has a detrimental influence on lower
limb function. Large diameter head total hip arthroplasty may theoretically have
a greater potential for restoring normal hip range of motion due to greater head-neck
diameter ratio, and hence provide better function compared to conventional or hip
resurfacing arthroplasty.

Method: At minimum one year follow-up, range of motion of the operated and contra
lateral hips was clinically assessed using digital photographs and bony landmarks
in a clinical comparative study. We assessed if 1) Large diameter head total hip
arthroplasty (55 patients) restores better hip range of motion compared to 28mm total
hip arthroplasty (50 patients) or hip resurfacing (60 patients) 2) Large diameter
head total hip arthroplasty achieves same hip range of motion as contra lateral normal
hips and 3) hip range of motion correlates with the WOMAC score.

Findings: The large diameter head total hip arthroplasty group had significantly
greater total arcs of motion (approximately 20 degrees), mostly due to an increase
of hip flexion and external rotation, but did not reach normal hip motion. The hip
range of motion showed significant correlation with the WOMAC score, especially the
flexion arc.

Interpretation: The better hip range of motion of large diameter head total hip arthroplasty
is likely due to the greater head to neck diameter ratio and hence seems to be the
best option to optimize range of hip motion and improve function after hip arthroplasty.